package y23.Oct;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Test30 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String str = in.nextLine();
        System.out.println(num(str));
    }

    public static char num(String str) {
        int[] count = new int[26];
        for (int i = 0; i < str.length(); i++) {
            count[str.charAt(i)-'a']++;
        }
        for (int i = 0; i < str.length(); i++) {
            if (count[str.charAt(i)-'a']==1){
                return str.charAt(i);
            }
        }
        return (char) -1;
    }


}
